U.S. and European officials Tuesday cautiously welcomed an Iranian proposal to end the decade-long standoff of its nuclear program, under which Iran would prove it’s not seeking to build a nuclear weapon in exchange for relief from crippling economic sanctions.

Iran’s foreign minister, Javad Zarif, delivered an hour-long PowerPoint presentation of what Iranian news reports said was a package of proposals to resolve the impasse. “It was very useful,” Michael Mann, the spokesman for the European Union, said afterward, without disclosing the content.

Experts then met for more than 2 1/2 hours to flesh out the proposals. This time the U.S. delegation, headed by Wendy Sherman, the undersecretary of state, issued a comment. “For the first time, we had very detailed technical discussions,” the statement said.

The talks were the first formal negotiations on the contentious issue since the election in June of President Hassan Rouhani, a reformist who succeeded hardliner Mahmoud Ahmadinejad. Rouhani won in a landslide after promising to do everything he could to resolve Iran’s overseas disputes and get the sanctions lifted.

Never miss a local story.

Sign up today for a free 30 day free trial of unlimited digital access.

Zarif’s deputy, Abbas Araqchi, told reporters the talks “were held in a highly positive atmosphere and the two sides were serious when speaking of their issues.” He said the Iranian proposal, which Zarif titled “an end to an unnecessary crisis, a beginning of a new horizon,” had the potential to bring about a breakthrough. “Progress would be very possible if there is goodwill on the other side,” he said, also refusing to give details.

Sherman and the U.S. delegation later met with Araqchi for an hour. The State Department called this the latest bilateral exchange – following President Barack Obama’s telephone call to Rouhani as the Iranian president was departing the U.N. General Assembly, and Secretary of State John Kerry’s meeting with Zarif during the U.N. session.

Mann, who spoke on behalf of the world powers represented in Geneva– the U.S., Russia, China, Britain, France and Germany – summed up that “it’s been a most useful day.” But having offered what perhaps was the most positive description in years of negotiations, he went out of his way to voice caution.

“I don’t want to get carried away,” he told reporters. “Obviously we’ve had a lot of mood music coming out of Tehran, a lot of statements that they’re willing to engage and be transparent. . . . The proof of the pudding is when they come forward with concrete proposals. They came forward with something this morning.” But then he added: “We need to work harder on it to get down to the nitty-gritty.”

The White House also treated Zarif’s proposal with caution. “I’m not sure there’s an offer,” spokesman Jay Carney said. “The Iranian delegation made a presentation,” and both sides “spent the day discussing the presentation.”

He refused to draw any conclusions.

“I would simply say that, after day one, we’re hopeful that we will make progress in Geneva,” he said.

Araqchi declined to give any details of the Iranian proposal, telling reporters: “It is confidential. And it is supposed to remain confidential.” Iranian news media for the most part reported the plan only in general terms.

Iran insists that its current program of building a stockpile of enriching uranium is intended strictly for peaceful use, but “certain things that they are doing now do not point in that direction,” Mann said.

Western governments, along with Russia and China, have focused on the size of Iran’s stockpile of enriched uranium, now 7.4 tons, according to the International Atomic Energy Agency, and its use of at least 10,000 centrifuges to produce still more enriched uranium. Most of that uranium is enriched to 5 percent, a level commonly used in many civilian applications, but Iran also has some 407 pounds of 20 percent enriched uranium, a far higher level of enrichment than is needed to power nuclear reactors and only a few steps short of the 95 percent level needed to build an atomic weapon.

The United States, prodded by Israel, Saudi Arabia and Persian Gulf countries, has been critical of Iran’s nuclear program for more than a decade. Iran made its first proposal for a resolution to the differences in 2003.

At the most recent previous talks, held in Kazakhstan in February, Iran offered to freeze the installation of centrifuges at an underground plant at Fordow and to suspend the enrichment of 20 percent uranium, but it demanded that in return all sanctions be lifted. The U.S. and its negotiating partners demanded a complete halt in all 20 percent enrichment; the transfer of the entire stockpile of 20 percent enriched uranium to a third country; and the suspension of all activities at the Fordow facility in exchange for sanctions relief in certain restricted areas – petrochemicals and precious metals.

At the current talks, the six world powers represented are demanding that Iran “verifiably profess” to the international community that it is not interested in or working toward a military nuclear program, Mann said. He added: “There is a long way to go.”